Nuclear magnetic resonance apparatus

ABSTRACT

An improved field homogenizing coil arrangement of a type having a generally tubular coil form extending about a test sample in an NMR apparatus is described. The coil arrangement includes a plurality of separate coils which are orthogonally related and which are formed of conductive segments positioned on opposite surfaces of the coil form and are conductively coupled through the form. The coils include arcuate-shaped segments positioned on one surface of the form and aligned normally with respect to a main polarizing field component, and rectilinear shaped segments positioned on an opposite surface of the form and extending in a direction substantially parallel to the main polarizing field component. The conductive segments are fabricated on the form by printed circuit techniques and the form is adapted to be rolled into a scroll-shaped configuration. With this arrangement, a relatively large number of field corrective coils are positioned on the form while overlapping rectilinear segments cancel undesired fields established by current return paths.

United States Patent R. F1 GENE/2H 70R SHIN I CURRE/V T 5 UPPL Y PrimaryExaminer-Michael J. Lynch Altorney- Edward R Hyde, Jr.

ABSTRACT: An improved field homogenizing coil arrangement of a typehaving a generally tubular coil form extending about a test sample in anNMR apparatus is described. The coil arrangement includes a plurality ofseparate coils which are orthogonally related and which are formed ofconductive segments positioned on opposite surfaces ofthe coil form andare conductively coupled through the form. The coils includearcuate-shaped segments positioned on one surface of the form andaligned normally with respect to a main polarizing field component, andrectilinear shaped segments positioned on an opposite surface of theform and extending in a direction substantially parallel to the mainpolarizing field component. The conductive segments are fabricated onthe form by printed circuit techniques and the form is adapted to berolled into a scroll-shaped configuration. With this arrangement, arelatively large number of field corrective coils are positioned on theform while overlapping rectilinear segments cancel undesired fieldsestablished by current return paths.

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PATENTEU MR 9 |97| SHEET DU [1F PATENTED MAR 9 l97| SHEET 1 3 HFPATENTEDHAR em lalltl 1 IIIIIIII'I ||l|| llllll SHEET 1 5 UP PATENTED m9 ran Q ANHQwNW WW RW SHEE 1 7 BF 1 J I n.. J. .H.. T. .u. .u. I: 7"u... I. L 4 r n- :I 1: nu Y J q-m H T. .T. ...n 2 I: :1 L." I." I I. T Tu... u n u. L. 1: u ..u. 3 T z Z v.... -F L Ft} J"- L-- :u-l 1: HI lrhmw 0% Na QR an wwm NW @N@ 0m MAW Nfih Nh PATENT ED MAR men .3. n v Qfis NUQLIEAR MAGNETIG RESONANCE APPARATUS This invention relates tonuclear magnetic resonance (NMR) apparatus adapted for analyzingsubstances by NMR techniques and more particularly to means forimproving the uniformity of a magnetic field established by suchapparatus.

NMR techniques have been employed in apparatus which function toidentify a substance by an atomic analysis of the substance. In general,a sample under investigation is positioned in a relatively intensepolarizing magnetic field and the Larrnour frequency of the atomicnucleus of the substance is determined in one form of apparatus bysuperimposing a less intense alternating magnetic field of varyingfrequency and sensing the frequency of sample resonance. Thegyromagnetic ratio of a sample element is then determinable from aknowledge of the intensity of the magnetic field and the Larmourfrequency. Identification of the particular element and isotope cantherefore be realized.

Microanalytical apparatus of the NMR type require a highly homogeneouspolarizing field in thespace occupied by the test sample in order thatan accurate identification of the nuclei can be made. Nonuniformities inthe field of an order as small as l can interfere with accurateidentification. Prior NMR arrangements have employed field correctingcurrent carrying electroconductors, also termed field homogenizing orshim coils which generate corrective fields for improving fielduniformity in the vicinity of the sample. The shim coils have previouslytaken the form of discs mounted on the pole faces of a permanent orelectromagnet. Means are provided for adjustment by the instrumentoperator of the amplitude of current flowing in the coils. In order thathomogenization of the field may be established by the instrumentoperator with facility, and in order to provide a sufficient degree offield homogenization, it is recognized that the individual shim coilsshould be orthogonally related in that the corrective fields generatedby the coils are independent, and, the coils should provide a relativelylarge number of order of correction. Orthogonality has been accomplishedby arranging a winding in a manner for providing that current flowingtherein generates an incremental magnetic field representable by aspherical harmonic of particular degree n and order m. Suitablehomogenization is accomplished by providing a plurality of such windingscorresponding to differing degrees and others. An arrangement of thistype is disclosed in my copending US. Pat. application Ser. No. 733,522,filed on May 31, 1968.

in a particular NMR apparatus the polarizing field is established by asuperconductive coil. Superconductivity advantageously provides apolarizing field having an intensity substantially greater than can berealized with present day permanent or electromagnet arrangements. Thesuperconductive coil is generally arranged as a solenoid and ispositioned in a housing along with cryogenic means for establishing arequired low operating temperature. This housing defines an annularopening extending therethrough and is exposed to roomtemperatureandpressure. The solenoid is arranged for establishing a polarizingfield in this opening and a sample to be analyzed is positioned in thisfield. I-Iomogenizing of the polarizing field is not readilyaccomplished with the disc type of homogenizing coil referred tohereinbefore because of the physical arrangement of the superconductingfield generating coil. l'lornogenizing coils in an NMR instrument ofthis type have been formed as a plurality of wire wound coils mounted ona tubular support form which is itself positioned within the annularopening and about the test sample. The coil support form then extends inthe direction of the main polarizing field component. A relatively highdegree of field homogeneity requires a relatively large number of wirewound coils. However, the provision of a relatively large number of suchwire wound homogenizing coils results in an arrangement which isrelatively bulky. In addition, the coils are difficult to fabricate,relatively difficult to accurately align, and the assembly is relativelyexpensive. For practical reasons then, the number of coils is therebylimited and the degree of homogenization provided by such a wound coilassembly is less than desirable.

Accordingly, it is an object of this invention to provide an improvedarrangement for homogenizing the polarizing field of an NMR instrument.

Another object of the invention is to provide an improved fieldhomogenizing arrangement wherein the coils are positioned about a sampleunder analysis.

Another object of the invention is to provide an improved fieldhomogenizing arrangement in an NMR instrument utilizing asuperconductive field establishing means.

A further object ofthe invention is to provide a printed circuithomogenizing coil arrangement wherein the coils are positioned about asample under analysis.

In accordance with the general aspects of this invention, parallelpositioned arcuate segments of homogenizing coils are positioned on onesurface of a printed circuit board which is formed in a manner forpositioning about a test sample. Rectilinear segments extending in aperpendicular direction to the arcuate segments are formed on the other,i.e., opposite surface of the printed circuit board. The segments onopposite sides of the printed circuit board are connected in a mannerfor providing that a plurality of electrically separate and orthogonallyrelated coils are formed for establishing corrective incremental fieldswhich can be represented by spherical harmonic functions. In onearrangement, the printed circuit board is formed for positioning in anannular opening of a housing of a superconductive field establishingmeans and about the test sample while in an alternative arrangement, theboard is fabricated with conductors which become superconductive atrelatively low temperatures and is positioned adjacent thesuperconductive solenoid and operated at substantially the samecryogenic temperature as the solenoid.

In accordance with another feature of this invention the printed circuitboard is formed of a material which renders the board flexible and isrolled into a scroll configuration. The rectilinear segments providingcurrent return paths are arranged on the printed circuit board in amanner for overlaying associated current return paths thereby cancellingundesired fields created by the current return paths in the direction ofthe main polarizing field.

These and other objects and features of the invention will becomeapparent with reference to the following specifications and the drawingswherein:

FlG. l is a diagram illustrating an NMR apparatus includingsuperconductive means for establishing a polarizing field and having afield homogenizing arrangement fabricated in accordance with features ofthis invention;

FIG. 2 is a diagram illustrating a printed circuit board for supportingfield homogenizing windings;

FIGS. 3A19A illustrate the locus of a spherical harmonic function, n, m,of a particular degree n and order m on a sphere at those points wherethe function vanishes;

FIGS. 33 through 198 illustrate the projection of the locus from anassociated sphere of FIGS. 3A through 19A on the surface of acylindrical body positioned about the sphere;

FIGS. 8049C illustrate a left side elevation view of the associatedcylindrical body of FIGS. 8B-I9B;

FIG. 8D is a perspective and more detailed view of the cylinder of FIG.3B illustrating the meaning of the double arrows and connections ofFIGS. 8B-I9B;

FIGS. 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30 and 31 illustrate flatcoil configurations which when rolled into the desired configurationwill establish the incremental fields corresponding to sphericalharmonic functions l,0; 2,0; 3,0; 4,0; 5,0; 2,1; 2,11; 3,1; 3,1; 3,2;3,2; 4,1; 4,l; 4,2; 4,2; and 4,3; 4,3 respectively, which willbediscussed in detail subsequently;

FIG. 32 is a diagram illustrating a building block configuration forgenerating incremental fields corresponding to tesseral (i.e., m il)harmonic functions; and

FIG. 33 is adiagram illustrating a circuit for cancelling ancillaryharmonics not cancelled by spacing of the electroconductors.

An inhomogeneity at a point in the polarizing field of an NMR instrumentcan be expressed as the sum of incremental

1. A field-homogenizing coil arrangement for a nuclear magneticresonance apparatus having a polarizing field component comprising: acoil form assembly having a body thickness thereof formed from anelongated sheet of insulating material sandwiched between first andsecond planar electroconductors, said body thickness rolled to provide agenerally tubular shaped coil form having a longitudinal axis thereofand an overlapping thickness; said coil form positioned about a testsample and having said longitudinal axis aligned with the polarizingfield component; a plurality of arcuate shaped electrically conductivesegments formed by said first electroconductor surface, each of saidarcuate segments laying in planes which are normal to the polarizingfield component of the apparatus; a plurality of rectilinear shapedelectrically conductive segments formed by said second electroconductor,said rectilinear segments extending in a direction generallyperpendicular to said arcuate segments and parallel to the polarizingfield component of the apparatus; means providing electricallyconductive coupling through said insulating material between arcuate andrectilinear segments to provide a plurality of separate electricallyinsulated field generating coils; and said arcuate and rectilinearsegments spaced on said form and intercoupled in a manner for providingthat each of said field generating coils establish orthogonally relatedhomogenizing fields when a current flows in the coils and at least someof the rectilinear segments of an individual coil are overlaid at saidoverlapping portion for carrying current in an opposite direction in amanner for effectively cancelling the magnetic fields established bythem.
 2. The homogenizing coil arrangement of claim 1, wherein each ofsaid coils is formed for establishing a corrective magnetic fieldrepresentable by a spherical harmonic of particular degree n and orderm.
 3. The field homogenizing coil arrangement of claim 2 wherein saidmagnetic potential from which said fields are derived comprises zonaland tesseral harmonics of successively higher degree.
 4. The fieldhomogenizing coil arrangements of claim 3 wherein said insulative formcomprises a central insulative material of a double-clad printed circuitboard and said segments are formed from conductive metal surfacesadhering to said insulative material, said double-clad board having athickness adapting said board to be rolled into a scroll shapedconfiguration.